Photography using silver halide has been most widely used because it provides excellent photographic properties, such as sensitivity and gradation, as compared to other methods such as electrophotography and diazo photography. Silver halide photography includes a color diffusion transfer process using light-sensitive element having a silver halide emulsion layer and image-receiving element having an image-receiving layer, laminated with either an alkaline processing composition spread in a layer between the elements, or being immersed in an alkaline processing solution. In recent years a new technique has been developed to easily and rapidly form images. In this process for forming images on a light-sensitive silver halide material, a dry process by heating or like means is used instead of the conventional wet process with a developing solution.
Examples of new color image formation processes by heat development are described in U.S. Pat. Nos. 4,463,079, 4,474,867, 4,478,927, 4,507,380, 4,500,626, 4,483,914, and 4,439,513, JP-A-58-149046, JP-A-59-48764, JP-A-59-65839, JP-A-59-71046, JP-A-59-87450, and JP-A-59-88730 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In these processes, a mobile dye is produced or released in correspondence or countercorrespondence to the reduction of a light-sensitive silver halide and/or an organic silver salt to silver by heat development and is then transferred to a dye fixing material.
Examples of a photographic material which can be used in such an image formation process include light-sensitive silver halides as described in JP-A-58-58543, JP-A-58-79247, and JP-A-61-238056, and a combination of a heat developable light-sensitive material containing a dye-providing substance which produces or releases a diffusible dye and a dye fixing material for fixing the dye transferred thereto.
In general, it is desired to provide a high pH in the reaction system for the development of a light-sensitive material, and for this a base is normally used in development.
In a wet image formation process such as the above-described development with a developing solution, such a base is normally incorporated in the processing solution (developing solution). In a simple dry image formation process by heating or like means it is normally necessary that such a base be incorporated in a light-sensitive material. However, such a process is disadvantageous in that if a base is incorporated in the light-sensitive material in an active form, it adversely affects the age stability of the light-sensitive material. For example, the base may adversely affect other components or may itself deteriorate during the storage of the light-sensitive material.
In order to solve these problems, it has been proposed that such a base be replaced by a precursor (base precursor) which is neutral or weakly basic during storage, but can produce a base during image formation. Various heat decomposition type base precursors have been studied and developed. Typical examples of such heat decomposition type base precursors include salts of carboxylic acids and organic bases. Examples of such a base precursor containing a salt of a carboxylic acid and an organic base include a salt of a triazine compound and a carboxylic acid as described in U.S. Pat. No. 3,493,374, trichloroacetate as described in British Patent 998,949, propiolate as described in JP-A-59-180537, and sulfonylacetate as described in JP-A-61-51139 and U.S. Pat. No. 4,060,420. These base precursors are subjected to decarboxylation of the carboxyl group contained in the carboxylic acid on heating. Accordingly, these base precursors are adapted to release an organic base during use (on heating).
In the improvement of such a base precursor, it is very important to attain both stability of the base precursor during storage and rapid decomposition of the base precursor (production of a base) during use. Conventionally, emphasis has been placed on the decarboxylability of the carboxyl group contained in the carboxylic acid to find improved carboxylic acids.
However, these base precursors fail to fully satisfy both requirements, i.e., stability during storage and rapid base production. They are also disadvantageous in that a heat developable light-sensitive material containing such a base precursor exhibits poor preservability and provides a low S/N ratio image density.